Satellite television receiving systems usually comprise an outdoor unit comprising a dish like antenna and an low noise block (LNB) amplifier, and an indoor unit, commonly referred to as a integrated receiver decoder (IRD). The IRD comprises the tuner and the signal processing section. The signal processing section of the IRD produces a plurality of timing signals or clocks, some generated by voltage controlled crystal oscillators (VCXO), used to tune the desired television signal to be displayed at the request of the user.
A VCXO oscillates at a certain frequency in response to a bit rate multiplier number (BRM). As the BRM is varied, the output frequency of the VCXO changes. Typically, during factory setup of the IRD, the appropriate BRM value is determined for a particular range of VCXO output frequencies and these BRM values are stored in a non volatile memory. However, physical attributes of the crystal and the environment, such as temperature, cut frequency, and age of the crystal, in which the IRD is used can affect the frequency output of the VCXO. One particular problem associated with VCXO drift is the ability of the IRD to generate the color subcarrier, where the VCXO must maintain a frequency of 27 MHz which is used by a PLL to generate 3.579545 MHz. Any discrepancy in this VCXO or the associated BRM could prevent the display from locking onto the colorburst signal thereby causing the video signal to be displayed in monochrome, partial color, and/or with color shifts.
To compensate for any discrepancy in the VCXO, the IRD microprocessor will track the interval between timestamps included in the incoming satellite signal and compare this interval with an interval similarly derived from a local clock based on the output frequency of the VCXO. If the compared intervals vary significantly, the microprocessor will alter the BRM to correct the VCXO clock output. Typically an acceptable variation for the VCXO clock is less than 15 ppm. As the physical attributes of the crystal or the environment change from those present during factory setup of the IRD, the time required for the microprocessor to adjust the BRM becomes more significant and increasingly unacceptable. Furthermore, if there is no reference timestamp present, this process becomes impossible and the microprocessor will be unable to adjust the factory set BRM value to compensate for deviations. It would be desirable to be able to correct for changes in the physical attributes of the crystal or the environment in the instances when no time stamps are available for the microprocessor to use for comparison.